This invention relates to a rotary transducer and is more particularly, but not exclusively, concerned with a rotary variable transducer, especially a rotary variable differential transducer.
It has been previously proposed to provide a rotary variable differential transducer having a body in which a primary and two secondary coils are wound on respective portions of a stator, such portions being equally-angularly spaced about the axis of rotation of a rotor mounted on a shaft in a plane perpendicular to the shaft. Each such stator portion extends over the same length of the rotor and carries an arcuate pole piece which is disposed close to the peripheral surface of the rotor. The rotor has an asymmetric pole piece which is arranged relative to the stator pole pieces associated with the primary coil and the secondary coils such that, upon angular movement of the shaft about its axis of rotation, the inductive coupling between the primary coil and one of the secondary coils increases whilst there is a corresponding decrease in the inductive coupling between the primary coil and the other of the secondary coils. The electrical output signals from the secondary coils can then be compared to give an indication of the angular position of the shaft relative to the body. Such a construction of transducer is relatively complicated to manufacture and can be difficult to assemble. Additionally, it does not have a particularly compact construction and the effective angular range is limited to .+-.60.degree. max approx. The linearity of signal output is dependent on the eddy current effects in the pole pieces. Use of solid magnetic pole pieces is desirable for constructional simplicity but eddy currents cannot be ignored in these. While eddy currents can be minimised by constructing the poles from thin laminations, their use makes assembly difficult and mechanical accuracy poor.
GB-A-2231161 discloses a rotary position transducer having a fixed primary coil which is substantially coaxial with a rotor shaft, and two fixed secondary coils symmetrically disposed on opposite sides of the primary coil axis with their common axis at right angles to that of the primary coil. The rotor shaft carries an asymmetrical rotor which provides inductive coupling between the primary coil and each of the secondary coils. The asymmetry causes the combined output of the secondary coils to vary, possibly linearly, with the angular position of the rotor. With such a construction it is also relatively difficult to achieve consistent results in manufacture and signal output is low due to poor magnetic coupling through the relatively large air gap between the rotor and the stator pole pieces. The linear range is also limited to about .+-.45.degree. to .+-.60.degree..